RU2613328C1 - Electrochemical method of methane concentration measurement in nitrogen - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: into a sample gas stream at the temperature from 450 to° 700C an electrochemical cell is placed with the cavity formed by the two discs of the oxygen-permeable solid electrolyte, on the opposite surfaces of the discs pairs of electrodes are located, DC voltage in the range of 600-1500 mV is supplied to the electrodes of one of the discs, with supplying the positive pole to an inner electrode, whereby the electrolysis of water vapour and pumping the oxygen resulting from electrolysis from the sample gas stream into the cell cavity is carried out by the electrochemical circuit: an outer electrode - solid electrolyte - an inner electrode; in the process of achieving a steady state, when the diffusive flux of methane oxidation products from the cell cavity becomes equal to the incoming flow of sample gas entering the cell, the limit current flowing through the cell is measured, and the methane concentration in nitrogen is determined by the limit current value corresponding to the content of oxygen spent for methane oxidation.

EFFECT: provision of possibility to simply and reliably measure the methane content in nitrogen.

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Description

The invention relates to the field of gas analysis, in particular to the registration and measurement of methane content in nitrogen, and can be used to solve technological problems, primarily related to the safety of technological equipment in which methane is stored and processed.

Creating in the atmosphere technological equipment and inert tanks from a mixture of methane and nitrogen is the most reliable and proven way to prevent fires and explosions during various types of work in the oil and gas industry.

The main methods for the determination of methane are thermochemical, interference and optical. Moreover, the thermochemical method allows you to analyze methane only in a gaseous environment, where there is an excessive content of air (oxygen). In an oxygen-free environment, for example, in a mixture of methane with nitrogen or another inert gas, this method is in principle inoperative. And the interference and optical methods based on the absorption of part of the spectrum of the light flux by methane make it possible to determine methane in its mixture with an inert gas.

The principle of operation of the interference gas analyzer is described in the source (V.G. Putilov, A.M. Petrova, R.Yu. Tolchenkin “Gas analyzer for monitoring methane released to the surface of the earth during the operation and liquidation of coal mines”, workshop No. 23 of the symposium “Miner's Week - 2007 ") [1]. This method is based on the selective absorption of infrared radiation by methane molecules in the wavelength region of 3.2 ÷ 3.6 μm, and in the gas analyzer, to compensate for the possible effects of moisture, dust and other factors capable of absorbing light, self-compensation two-beam optical design. It measures the intensity of two rays passing along the same optical path, the wavelength of one measuring beam being in the absorption region and the other of the reference beam in the region where there is no absorption. The emitters and photodetectors used in the gas analyzer change parameters over time, with changes in temperature, as well as during aging. To automatically compensate for these changes, two more beams were introduced into the optical circuit that did not pass through the analyzed gas mixture. The method is hardware-complicated, time-consuming and requires qualified maintenance.

The optical method for determining methane is implemented in the well-known remote methane detector (RU 2029287, publ. 02.20.1995) [2]. The method consists in the fact that the laser radiation is alternately supplied to the mirrors, with the help of which it is divided into reference and probe beams. The latter, passing through the investigated region of space, fall on the topographic diffuser. The backscattered radiation is collected by the telescope and sent to the photodetector 7 located at its focus, connected to the input of the amplifier. The output of the amplifier 9 is connected to the input of an analog-to-digital converter. The reference beams using mirrors are fed to a photodetector connected to the input of the analog-to-digital converter, the outputs of which are connected to the input circuits of the microcomputer. The probe beams, alternately passing through the investigated region of space, are absorbed differently by methane, as a result of which the radiation scattered back by the topographic scatterer at these wavelengths is differently attenuated. This radiation, collected by the telescope, is converted by means of a photodetector into an electrical signal, and then, after amplification, it is converted using an analog-to-digital converter into a digital code.

The method is characterized by a complex hardware design, the complexity, the need for qualified maintenance personnel.

The objective of the present invention is to provide a method that allows a fairly simple and reliable measurement of the methane content in nitrogen.

To solve this problem, an electrochemical method for measuring the concentration of methane in nitrogen is proposed, which consists in placing an electrochemical cell with a cavity formed by two disks of an oxygen-conducting solid electrolyte on opposite surfaces in the flow of the analyzed gas at a temperature of 450 to 700 ° C. disks are arranged along a pair of electrodes; a direct current voltage of 600–1500 mV is applied to the electrodes of one of the disks with a positive pole supplied to the internal electrode; As a result, water vapor is electrolyzed and oxygen obtained as a result of electrolysis is pumped from the analyzed gas stream into the cell cavity along the electrochemical circuit: the outer electrode is a solid electrolyte - the inner electrode, in the process of reaching a stationary state when the diffusion flow of methane oxidation products from the cell cavity becomes equal the incoming flow of the analyzed gas entering the cell, measure the limiting current flowing through the cell and the value of the limiting current corresponding to zhaniyu oxygen spent in the oxidation of methane, the concentration of methane in nitrogen.

When a DC voltage is applied to the electrodes of one of the disks in the range of 600-1500 mV with a positive pole supplied to the electrode inside the cell, the water vapor in the analyzed gas is electrolyzed and the oxygen obtained as a result of electrolysis is pumped from the analyzed gas stream into cell cavity along the electrochemical circuit: outer electrode - solid electrolyte - inner electrode. In the cavity of the cell, the process of oxidation of methane with oxygen pumped into the cavity will proceed in accordance with the reaction:

When the DC voltage reaches a value in the range of 600-1500 mV, the current stabilizes and stops growing with increasing voltage. The resulting current is the limiting current, and its value is due to gas exchange between the analyzed medium and the gas in the cell cavity. The value of the limiting cell current is limited by the diffusion barrier - capillary and is related to the concentration of ammonia (Ivanov-Shits, I. Murin, Solid State Ionika, Volume 2, St. Petersburg (2010) SS. 964-965) by equation (2):

where D _{(methane inert gas)} is the diffusion coefficient of ammonia in an inert gas;

X _(methane) is the molar fraction of ammonia in an inert gas;

S is the cross-sectional area of the capillary, mm ² ;

P is the total pressure of the gas mixture, atm;

T is the analysis temperature, K;

L is the length of the capillary, mm

In accordance with equation (2), it is quite easy to calculate the methane content from the measured value of the limiting current I _{L (methane is an inert gas)} .

In this case, another solid electrolyte disk with electrodes deposited on its opposite surfaces performs the function of monitoring the operability of the electrochemical cell by the magnitude of the EMF, which, in accordance with equation (3), reflects the change in the partial pressure of oxygen inside the cell relative to the partial pressure of oxygen in the analyzed gas stream:

where E is the cell EMF, mV;

R is the universal gas constant equal to 8.314 J / mol-K;

T is the temperature, K;

4 - the number of electrons involved in this electrode reaction;

F is the Faraday number equal to 96,500 C / mol;

P (O2) ¹ is the partial pressure of oxygen in the cell cavity;

P (O2) ² is the partial pressure of oxygen in the flow of the analyzed gas.

On the one hand, the generated EMF value confirms that the current limit was established on the disk with the electrodes to which the voltage was applied - the EMF value also stabilizes, and also indicates a possible depressurization of the electrochemical cell. During depressurization of the electrochemical cell, the EMF will drop to zero, because the ratio P (O ₂ ^l ) / P (O ₂ ² ) becomes equal to unity.

A new technical result achieved by the claimed method is to obtain the possibility of measuring the methane content in nitrogen in a wide range of temperatures and concentrations while monitoring the operability of the electrochemical cell during the measurement process.

The invention is illustrated by drawings, where in FIG. 1 shows an electrochemical cell for implementing the method; in FIG. 2 - dependence of the current of the disk with the electrodes, to which voltage is supplied and the EMF of the disk with the electrodes from which the EMF is removed, from the applied voltage for gas CH ₄ (5%) + N ₂ at a temperature of 550 ° C; in FIG. 3 - dependence of the limiting current of the electrochemical cell on the voltage and concentration of CH ₄ ; in FIG. 4 - dependence of the limiting current of the electrochemical cell on the methane concentration.

An electrochemical cell for implementing a method for measuring methane in nitrogen consists of two disks 1 and 2, made of an oxygen-conducting solid electrolyte of the composition 0.9 ZrO ₂ + 0.1Y ₂ O ₃ . On the opposite surfaces of each of the disks 1 and 2, two outer electrodes 3 and 4 and two inner electrodes 5 and 6 are located. The disks 1 and 2 are interconnected by a gas tight sealant with the formation of an internal cavity 7. Between the disks there is a capillary 8. A voltage is applied to the electrodes 3 and 4 of the disk 1 are carried out from a DC voltage source, and plus is supplied to the internal electrode 4. The current arising in the cell circuit is measured with an ammeter (A). The electrochemical cell itself is placed in a stream of analyzed gas, which washes its outer surface and flows through the capillary 8 into the inner cavity 7. Under the action of a DC voltage applied from a source (IPT) to electrodes 3 and 4, oxygen is pumped through a solid oxygen-conducting electrolyte from the analyzed gas into the internal cavity of the cell 7. In the cavity 7, the incoming oxygen interacts with methane. The resulting interaction products, in accordance with equations (1-2), are exchanged through capillary 8 with the analyzed gas. In this case, capillary 8 is a diffusion barrier that limits this gas exchange flow. This current flow will correspond to the cell current. When the applied voltage is reached within 600-1500 mV, the gas exchange between the cell cavity and the analyzed medium is stabilized and the limiting diffusion current I _{L (methane - nitrogen) is} established in the circuit, which is measured using an ammeter (A). Using equation (2) from the value of the measured I _{L (methane - nitrogen),} it is possible to determine the value of X _(methane) , i.e. concentration of methane in nitrogen.

Thus, the claimed method allows to measure the methane content in nitrogen by means of an amperometric cell with an oxygen-conducting solid electrolyte. Since the properties of inert gases are identical, the claimed method can be applied to measure the methane content in other inert gases.

Claims (1)

The electrochemical method for measuring the concentration of methane in nitrogen, which consists in placing an electrochemical cell with a cavity formed by two disks of an oxygen-conducting solid electrolyte in the stream of the analyzed gas at a temperature of 450 to 700 ° C; a pair of electrodes are located on opposite surfaces of the disks , a direct current voltage of 600-1500 mV is applied to the electrodes of one of the disks with a positive pole supplied to the internal electrode, whereby the vapor is electrolyzed odes and pumping the oxygen obtained as a result of electrolysis from the stream of the analyzed gas into the cell cavity along the electrochemical circuit: the outer electrode - solid electrolyte - the inner electrode, in the process of reaching a stationary state when the diffusion stream of methane oxidation products from the cell cavity becomes equal to the incoming stream of the analyzed gas, entering the cell, the limiting current flowing through the cell is measured and by the magnitude of the limiting current corresponding to the oxygen content spent on oxidation methane, the concentration of methane in nitrogen.

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